Aromatization of straight run naphthenic gasolines



couvaRaon, MOLE U1 Aug. 12, 1958 I. G. IYNIXON v AROMATIZATION OFSTRAIGHT RUN NAPHTHENIC GASOLINES Filed Aug. 23, 1955 2 Sheets-Sheet 1CYCLOH EXANE METHYLCYCLOPENTANE TEMPERATURE, c

Fiql

lnven'l'or; \vor 612:9 Nixon I. G. NIXON Aug. 12, 1958 AROMATIZATION OFSTRAIGHT RUN NAPHTHENIC GASOLINES 2 Sheets-Sheet 2 Filed Aug. 23, 1955lnvenTor: \vor rcu Nixon 23M PIQZMPW owNi/ibmmo United. StateAROMATIZATION F STRAIGHT RUN NAPHTHENIC GASOLINES Ivor Gray Nixon, TheHague, Netherlands, assignor to Shell Development Company, New York, N.Y., a corporafion of Delaware pressure.

REFORMING It is well known that various gasolines having pooranti-knockcharacteristics can be improved by a catalytic reforming treatment. Inthe usual catalytic reforming process the gasoline is passed in vaporphase in contact with one of a number of dehydrogenation type reformingcatalysts such for example as molybdenum oxide supported on an aluminacarrier. The operation is usually carried out in the presence of addedhydrogen. In this process, the main improvement is due todehydrogenation of cyclohexane homologues to the corresponding aromatichydrocarbons. Also, if the hydrogen pressure is not too high, some minoramount of aromatic hydroe carbons is produced from paraifin hydrocarbonsby dehydrocyclization. The improvement in anti-knock properties obtainedby these processes increases with increasing severity of the treatingconditions until the available hydroaromatic naphthenes aresubstantially completely converted to aromatic hydrocarbons. The F-2octane number of the product at this point is generally between 70 and80. A further improvement in octane number may be obtained by increasingthe severity of the treating conditions but this further increase isobtained largely by concentration of the aromatic hydrocarbons alreadyformed through cracking out the less refractory nonaromatichydrocarbons; such further improvement is, therefore, accompanied by alarge loss in yield of liquid product. In commercial practice an 80%yield of a product of about 80 F-2 octane number is considered to beabout the upper limit for economical operation. The high octane numbersobtainable by this process are only possible because the process isregenerative, i. e., the catalyst is regenerated after each short periodof use. If it is desired to operate in a non-regenerative manner,relatively mild conditions must be used and the attainable improvementis less. By careful fractionation of the product a fraction suitable foruse as aviation base stock can be obtained but the yield is poor. Asimilar product can, of course, be obtained by severe thermal reformingfollowed by fractionation but the yield is so low that the process isclearly of no practical interest under normal conditions.

PLATFORMING Recently a reforming process known as platforming has comeinto use for reforming motor 'gasolines. In this process gasolines ofpoor anti-knockcharacteristics are improved by treatment in the vaporphase and in the presence of hydrogen under pressure with a catalystconsisting of a cracking catalyst promoted with platinum. Aside fromthetotally different catalyst used, this process differs from theprevious reforming process in that it is non-regenerative, i. e., asingle charge of the catalyst is out regeneration.

hydrocarbons and hydrogen are continuously passed.

atent "ice - cyclopentane homologues, which are usually present inappreciable concentrations, are likewise converted to a substantialextent to aromatic hydrocarbons. Also, some of the normal paraflins areisomerized to isoparafiins and this contributes to a minor extent to theimprovement.

While platforming has been rather broadly described it is limited inpractical application to the treatment of certain feed stocks. This isdue to the nature of the reactions which are known to take place. On theone hand, the final boiling point of the feed must not be too high sincehigher boiling materials tend to coke the catalyst. A final boilingpoint around the end point of motor gasoline is generally consideredabout the maximum. On the other hand, the process is most amenable tothe higher boiling gasoline constituents. It is therefore generallyapplied to the high boiling fraction of the gasoline. This is duelargely to two factors. The first is that in platforming hydrocrackingis one of the most important reactions. When treating 200400 F. naphthathis hydrocracking leads to the production of high octane componentswhich still boil in the gasoline range and therefore tend to increasethe volumetric yield. If lower boiling fractions are treated or includedin the feed this hydrocracking leads to the production of materialswhich boil below gasoline or in the lighter fraction thereof and must beremoved to reduce the volatility to the usual range thereby considerablydecreasing the volumetric yield. The second is that the improvement inthe octane number which may be obtained by platforming sharply declinesas the boiling range of the feed islowered so that a point is soonreached where the minor improvement is insufiicient to justify theconsiderable cost of the platforming operation.

In motor gasoline production it is therefore the practice to platformonly the high boiling portion of straight run gasoline boiling betweenabout ZOO-400 F. In the production of aviation gasoline, which has alower boiling range than motor gasoline, a fraction of somewhat lowerboiling range is treated, e. g., 185-270 F. In either case it isconsidered undesirable to include hydrocarbons boiling below hexanes andit is therefore the practice to top the straight run gasoline to removeall such material boiling below at least about F. Thus, for exampleFulton (Petroleum Refiner, vol. 29, pp. 109-112 (1935)) describes theplatforming of various natural gasoline fractions and states thatplatforming is useful only for the treatment of the hexanes plusmaterial and does not apply to the treatment of butanes and pentanes.

The present practice of excluding pentane in the platforming of straightrun naphthenic gasoline fractions is based on the followingconsiderations. Pentane, since it contains only 5 carbon atoms, cannotbe converted by dehydrogenation or dehydrocyclization to any aromatichydrocarbon. Hydrocracking is an important reaction in platforming andthe catalyst is consequently compounded to produce substantialhydrocracking. The hydrocracking of pentane can lead only to theproduction of undesirable gases, to lowering of the volumetric yield,and

used continuously over an extended period of time withto consumption ofvaluable hydrogen.

The catalyst is, therefore, employed ent in straight run distillates isnot pure normal pentane 7 in the form of one or more fixed beds throughwhich the 7 7 but a mixture of normal pentane and lsopentane.

The pentane pres- The only advantage which could be expected byincluding F-Z-O octane number of the treated C fractiontherefore' wouldbe less than about 76 i. 3 which is lower than the octane number of thehigher boiling portion of the platformate. Its presence would thereforetend to lower the octane number of the total product.

The exclusion of lower boiling straight run components, and especiallypentane, from the platforming feed is based primarily on the abovereasons. Experimental runs using straight run feeds of initial cuflpointto include the small amounts of normally occurring pentanes could not beexpected to show any beneficial results for the above mentioned reasonsand also because the pentane content of such feeds would not amount tomore than about 7%.

ADDITION OF PENTANE Contrary to the belief and teachings hithertoadvanced regarding the desirability of excluding pentane in theplatforming feed, it has now been found that the inclusion of sizeableamounts of pentane has a totally un-' expected beneficial effect. Thus,it has been found that in the presence of sizeable amounts of addedpentane the elficiency of conversion of the naphthenes to aromatics(whichis the chief octane-raising reaction in the process) isappreciably increased. It was at one time thought that this improvementwas due to the relatively high heat carrying capacity of the addednormal pentane which acted as a diluent and possibly also to a smallextent by the small heat of the limited isomerization of part of thepentane, which isomerization is slightly exothermic and is known to takeplace.

Thus, as pointed out, when the reforming operation-is carried out withthe mentioned acid platinum catalyst, the C -ring naphthenes which arenormally present in about equal amounts with the (Z -ring naphthenes arelargely converted to aromatics by dehydroisomerization. The totalreaction is, therefore, quite endothermic. It is, in fact, soendothermic that it is difficult to carry out the process without atemperature drop in the catalyst bed. The major amount of theendothermic heat supplied must be supplied by preheating the reactantfeed stream. However, the maximum temperature at which the material canbe preheated without causing excessive thermal cracking and/ orexcessive hydrocracking in the forepart of the catalyst bed is strictlylimited. In normal operation with the preferred and enerally usedadiabatic reactors the temperature drop through the catalyst bed isusually over 50 C., and may be as much as 110 C. or more. (See BritishPatent No.f662,002.)

The main desired reaction is the conversion of the naphthenichydrocarbons in the feed to aromatic hydrocarbons and this is anequilibriumlimited reaction which is influenced by the temperature. Themaximum degree of this conversion is, therefore, limited by the'equivlib'riurn conversions of cyclohexane and methylcyclopentane to benzeneplotted against the temperature at a pressure of 30 atmospheres and ahydrogen-to-hydrocarbon mole ratio of 3. The curves are shifted towardthe right if the pressure is increased and are shifted downward slightlyif the hydrogen-to-hydrocarbon mole ratio is increased. The curves forthe equilibrium conversions of the C and C naphthenes are not identicalbut from the limited data available, they are of similar shape andrelative positions.

Referring to Figure 1, it will be noted that under the moderate pressureof 30 atmospheres, and with the low -hydrogen-to-hydrocarbon mole ratioof 3 it is possible to obtain over conversion of the cyclohexane at atemperature of 475 C. However, the maximum possible conversion of themethylcyclopentane is less than 60%. Such conversions, if attainable,would not be bad. However, if 'a temperature drop of 50 C. takes placein the catalyst bed, the exit temperature is 425 C. and at thistemperatureit is seen that the maximum possible conversions are only 53%and 13%, respectively. The usual means for overcoming this disadvantageis to reheat the product back to a high temperature and retreat it. Inthe second treatment, the temperature drop is less and, consequently, ahigher outlet temperature with a higher limiting equilibrium ispossible. Even two such treatments are usually not sufficient to obtaina really efficient conversion and in commercial practice three and oftenfour such treatments are normally used.

If appreciable amounts of normal pentane are added to the feed thetemperature drop, although still appreciable will be less and, as willbe seen from the curves in the graph, this could result in a morefavorable limiting equilibrium conversion.

The above explanation for the improved results appears plausible but itdoes not fit the facts since the improved conversions have now beenobtained under carefully controlled temperature conditions where thisheat effect is negligible. Thus, for some unexplained reason theinclusion of appreciable amounts of added pentane results in increasedconversion and conversion efiiciency, particularly with respect to the C-ring aromatic precursors, even under carefully controlled temperatureconditions and even at the same base feed throughput rate (henceincreased liquid hourly space velocity).

This is illustrated by the data in the following Tables I, -II, and IIIin which are shown the results obtained in platforming the same feedstock containing different amounts of added normalpentane. In order toshow the effect most clearly with the least difficulties anduncertainties due to analytical errors these experiments were carriedout with a. synthetic feed prepared with relatively pure startingmaterials. To this base stock various amounts of normal pentane wereadded to give feeds having. the compositions shown in the followingTable I.

Table I Feed Pound moles per hundred pounds of feed Normal PentaneMethylcyclopentane;

Tolueneofzeasmay mol) 0. 5198 (437 mol) 0. 8316 66 mol 0.4040 0 0.31250.1s9s

The platforming of these blends was carried out at a liquid hourly spacevelocity of 2 with a commercial platforming catalyst and with recycledhydrogen. There were 2 reactors in the series, each provided with meansfor careful temperature control. The pressures in the reactors were 325and 275 p. s. i. g. The pertinent results are shown in the followingTable II.

It will be noted that the equilibrium conversion of methylcyclopentane(MCP), which is the most sensitive indicator in this case, is the samewithin experimental limits in all three cases so that loss of conversiondue to an unfavorable equilibrium at lower temperatures is excluded. Itwill be noted furthermore that the percent of equilibrium conversionobtained, i. e. the percentage approach to the equilibrium, was high inall cases and increased significantly with increasing dilution withpentane. Thus, the mols of benzene formed per mol of methylcyclopentanecharged increased from 0.456 to 0.615 or an increase of 33%. Themethylcyclohexane was in each case converted substantially completely totoluene. These data show that the improved results are not dependentupon any temperature effect and that they are due primarily to improvedconversion of the C -ring naphthenes, which, as pointed out, constituteapproximately half of the aromatic precursors in the usual feed stocks.

The above results were obtained by operating with a I constant totalfeed space velocity (LHSV=2). This advantage however is still maintainedif the space velocity, based on the total feed, is increased so that thesame amount of base feed is processed. The corresponding results on thisbasis (calculated from the same data using previously determinedconversions versus space velocity correlation curves) are shown in thefollowing Table III.

Table III Concentration normal pentane, mol percent- 23 43 66 Liquidhourly space velocity 2 2. 5 3. 5 Conversion MOP to benzene, percent 4248 52 Relative benzene production rate, welght/ tirns 1. O 1. 14 1. 24

In order to produce aviation base stock of high quality,

a narrow boiling straight run fraction is separated for treatment. Thecut points should be chosen so that the majority of the usefulnaphthenes are concentrated. On the other hand, the end point should below enough to meet the boiling range requirements of the compositecontaining the isopentane. For the reason stated above, the Chydrocarbons should be excluded as far as practicable. The fraction ca.85 C. to ca. 130 C. (cut point temperature) is suitable for aviationbase stock production. All straight run gasoline fractions of thisboiling range are, of course, not suitable. The straight run gasolinefractions should be one in which naphthenic hydrocarbon content is high,preferably at least 50%. The

naphthene content of the fraction is dependent upon the naphthenicity ofthe petroleum from which it is derived.

The amount of added normal pentane must be considerable. It should beborne in mind that the equilibrium between norm-a1 pentane andisopentane is not particularly favorable under the reaction temperatureconditions and also that complete conversion to the equilibrium is notusually obtained. The amount of normal pentane required is, therefore,considerably above the normal pentane concentration in the straight rungasoline cuts regardless of their initial boiling point. In general, theamount of normal pentane required to be added is between about 10% and70% of the narrow boiling fraction to be treated and preferably theamount is between 25% and 70%.

Upon completion of the conversion the normal pentane is removed. Therecovered normal pentane is advantageously recycled. On the other hand,it is important that no appreciable amount of isopentane be recycled.

The process of the invention will be described in more detail inconnection with Figure II of the accompanying drawing whichschematically shows a flow diagram of one application of the process forthe production of an aviation base stock.

I Referring to Figure 11, a debutanized straight run gaso-' line derivedfrom a naphthenic petroleum and entering via line 1 is passed tofractionating column C Column C is operated to take overhead all of thepentanes. The overhead product is essentially normal pentane butcontains a small amount of isopentane. The bottom product from column Cis passed by line 2 to fractionating column C Column C is operated totake overhead all of the hexanes. The overhead product is withdrawn byline 3. The bottom product from column C is passed by line 4 tofraction-ating column C Column C is operated to take overhead thefraction desired for the production of aviation base stock. As pointedout, the recommended cut points for this fraction are C. and C. Thebottom product from column C is withdrawn from the system by line 5.

The pentane fraction from column C is combined with an isopentane-richfraction from line 6, later to be described, and the mixture passed tofractionating column C Column C is operated to separate isopentane fromnormal pentane. The overhead isopentane fraction is withdrawn by line 7and combined with the reformed stock as later described. The normalpentane fraction withdrawn by line 8 is combined with the overheadfraction from column C As previously pointed out, the amount of pentanenormally occurring in straight run gasolines is not suflicient to supplythe needs for the process. Normal pentane or a pentane fraction from anexterior source, e. g., from a straight run gasoline of lownaphthenicity is therefore introduced by line 9. The mixture of theselected straight run fraction, the recycled normal pentane, and thenormal pentane introduced by line 9, is mixed with recycled hydrogenfrom line 10 and passed to the platforming unit indicated simply in thefigure by the labelled rectangle. In the platforming unit the materialis preheated to a reaction temperature of about 890 F. and then passedthrough a bed of catalyst consisting of from about 0.1 to about 0.4% ofplatinum supported upon a cracking catalyst such as alumina treated withhydrofluoric acid or an equivalent fluo-rinating agent, or a syntheticsilica-alumina composite cracking catalyst. The pressure is normallybetween about 200 and 700 p. s. i. g. The space velocity, i. e. contacttime, is adjusted such that the effiuent is substantially at equilibriumfor the exit temperature. The vaporous effiuent is cooled somewhat tocondense the major portion of the normally liquid hydrocarbons and thenpassed by line 11 to column C The purpose of column C is to separateproduct gas consisting essentially of hydrogen while retaining all ofthe isopentane in the bottom product. To

this end cooled higher boiling product fromline 12 is passed into thetop of column C This cooled higher boiling product passing downwardthrough the column eifectively removes isopentane from the overhead product. The product gas consisting essentially of hydrogen is recycled tothe platform unit by line as described. The amount of hydrogen thusrecycled is generally between about 2 and about 10 moles per mole ofhydrocarbon feed to the platforming unit. Any product gas in excess ofthis amount may be withdrawn by line 13. The bottom product from columnC is passed by line 14 and valve 15 to separator 16 which normallyoperates at a lower pressure than column C Low pressure gas releasedupon reduction of the pressure is withdrawn by line 17. The liquidproduct from the separator is passed by line 18 to fractionating columnC Column C is operated to take overhead all of the pentanes. The bottomproduct from column C is a highly aromatic material consisting almostexclusively of C and C hydrocarbons. Part of this cooled product isrecycled to column C as described. The remaining part is combined withthe overhead fraction from column 0.; consisting substantially ofisopentanes. The mixing containing the correct ratio of isopentane toreformed stock to provide the required volatility is withdrawn by line18 as product aviation base stock. The pentane fraction withdrawn asoverhead product from column C is recycled back to column O; asdescribed. The volatility of the aviation base stock withdrawn by line18 depends upon the amount of the isopentane fraction recovered as topproduct from column C The desired correct amount is obtained byadjusting the amount of normal pentane from an outside source introducedby line 9. In a typical case the 85130 C. straight run fractionseparated had the following composition:

Percent by weight Naphthenes 62 Aromatics 3 Paraffins 35 The amount ofnormal pentane required for the distillate was 35 parts by weight foreach 65 parts by Weight of the distillate. This mixture was treated witha commercial platforming catalyst at a space rate of 2.5 kg. per literof catalyst per hour at 430 C. and atmospheres pressure. The amount ofhydrogen used was 1000 liters (standard conditions) per kilogram of thehydrocarbon feed. There was no noticeable cracking of the pentanes. Thepentanes were separated from the product and separated into normalpentane (55%) and isopentane (45%). The isopentane was blended back withthe reformate and was just suflicient to provide the necessaryvolatility.

I claim as my invention:

1. In the production of gasoline from straight run naphthenicdistillates boiling in the gasoline boiling range the improvement whichcomprises adding to the straight run distillate from 25 to about ofnormal pentane, reforming the resulting mixture in the presence of addedhydrogen with an acid platinum catalyst, and fractionating the resultingproduct to separate normal pentane.

2. In the production of gasoline from straight run naphthenicdistillates boiling in the gasoline boiling range the improvement whichcomprises removing from the distillate all of the material boiling upthrough hexane and material boiling above the gasoline boiling range,adding to the remainder from 25 to about 70% of the amount thereof ofnormal pentane, reforming the resulting mixture in the presence of addedhydrogen with an acid platinum catalyst, and fractionating the resultingproduct to separate normal pentane.

3. In the production of gasoline from straight run naphthenicdistillates boiling in the gasoline boiling range the improvement whichcomprises fractionating the naphthenic distillate to remove therefrommaterial boiling up through hexane and material boiling above about 130C. leaving a naphthenic fraction boiling between about C. and C., addingto said naphthenic fraction from 25 to about 70% of the amount thereofof normal pentane, reforming the resulting mixture in the presence ofadded hydrogen with an acid platinum catalyst and fractionating theresulting product to separate normal pentane.

4. Process according to claim 3 further characterized in that isopentaneis separated from the pentanes from the straight run naphthenticdistillate and from the pentane recovered from the reformed product,said separation being effected together by fractional distillationwhereby the isopentane present in the straight run distillate isrecovered with the isopentane from the reformed distillate and thecombined isopentane is re-mixe-d with the depentanized reformed product.

References Cited in the file of this patent UNITED STATES PATENTS Read:.Petroleum Refiner, vol. 30 (March 1951), pp. l30-l36.

2. IN THE PRODUCTION OF GASOLINE FROM STRAIGHT RUN NAPHTHENIC DISTILLATES BOILING IN THE GASOLINE BOILING RANGE THE IMPROVEMENT WHICH COMPRISES REMOVING FROM THE DISTILLATE ALL OF THE MATERIAL BOILING UP THROUGH HEXANE AND MATERIAL BOILING ABOVE THE GASOLINE BOILING RANGE, ADDING TO THE REMAINDER FROM 25 TO ABOUT 70% OF THE AMOUNT THEREOF OF NORMAL PETANE, REFORMING THE RESULTING MIXTURE IN THE PRESENCE OF ADDED HYDROGEN WITH AN 